This application claims priority of European Patent Application No. 99309580.1, which was filed on Nov. 30, 1999.
The invention relates to an apparatus for receiving electromagnetic signals, with the capability for tracing free or occupied radio connecting channels, and to a method which can be used, in particular, with the apparatus.
In the course of a continuously increasing need for communication and mobility, transmission techniques using electromagnetic signals are of particular interest. The rapidly expanding field of mobile radio which normally operates in the Global System for Mobile Communications Standard (GSM), represents an important application of such transmission techniques. So-called base stations are used as connecting nodes in order to set up a connection between two mobile radio subscribers or one mobile radio subscriber and a communication subscriber in the fixed network. In order to ensure that simultaneous transmission and reception are guaranteed at all times on an existing connection the GSM Standard provides separate frequency bands for transmitting and receiving the electromagnetic signals. For example, base stations using the GSM900 Standard transmit in the 925 to 960 MHz frequency band, and receive in the 880 to 915 MHz frequency band. Base stations are typically split into a transmission path and a reception path, for this purpose. Each frequency band is, furthermore, split into a number of channels, which typically have a width of about 200 kHz. Relatively large base stations (so-called macro base stations), as are used to cover large regions, use a predetermined frequency plan in order to avoid simultaneous and interfering use of the same channel by two base stations located close to one another or two transmission/reception cells within the same base station.
Furthermore, the increased use of mobile radio systems within buildings has resulted in a requirement for simple, small and cheap base stations, in order to ensure network coverage without any gaps. For this purpose, a plurality of such base stations, so-called xe2x80x9cpico base, stationsxe2x80x9d which typically have a transmission power of about 100 mW, are used in the building. In order to avoid the complex, predetermined frequency plan, pico base stations frequently have a device for scanning the transmission channels in order to find a free channel, on which it is then possible to transmit, before setting up a radio link to, for example, a mobile radio telephone. Normally, this also ensures that a free reception channel is available, since the channels from the transmission band and reception band are associated with one another in pairs. In this case, the separation between the pairs of associated transmission and reception channels is the same for all the channels used.
A known receiving apparatus having a scanning capability for the GSM900 Standard contains a mixer, which converts received signals to an intermediate frequency of, for example, 45 MHz by means of an oscillator frequency. In the reception mode, the oscillator is tuned to a frequency that is 45 MHz higher than the received frequency, in order to produce a sideband at the intermediate frequency by heterodyning with the received frequency. The signals at this intermediate frequency are then filtered by means of a bandpass filter, which has a bandwidth of one channel width, and, in the reception mode, are passed to a receiver. In the scanning mode, the oscillator tuning is varied and the measurement is carried out at the receiver to determine whether the respectively scanned frequency is, for example, occupied by another base station. In this concept, in the reception mode, the oscillator thus operates in the frequency range from 925 MHz to 960 MHz and, correspondingly in tracing mode, operates in the frequency range from 970 MHz to 1005 MHz, since the signals at the transmission frequency are also converted to the intermediate frequency. In consequence, the oscillator disadvantageously has to cover the entire frequency range from 925 MHz to 1005 MHz, that is to say has to have a bandwidth of 80 MHz, even though a bandwidth of only 35 MHz is required to cover the reception band in the GSM900 concept. This oscillator bandwidth, which is increased by a factor of about 2.3, has a disadvantageous effect on its noise characteristics.
Furthermore, when changing from the reception mode to the scanning mode or vice versa, the oscillator is tuned through 45 MHz in order to test the transmission channel associated with the reception channel for freedom or occupancy and this, disadvantageously, takes a relatively long time of, typically, 1 to 2 ms.
Furthermore, it is disadvantageous that the filter bandwidth for scanning and reception cannot be chosen independently of one another, that is to say, in particular, only one transmission channel can be tested at a time.
The invention is thus based on the object of providing an apparatus for receiving electromagnetic signals with an improved capability for tracing free or occupied channels and which allows, in particular, a fast changeover from the reception mode to the tracing mode, and vice versa.
The object is achieved in a surprisingly simple manner just by the features of claim 1 or 17. Advantageous developments of the invention are the subject matter of the dependent claims.
The apparatus comprises an antenna for receiving electromagnetic signals, and a selection device, for example a diplexer, using which predetermined frequency ranges can be selected. The selection device is associated with the antenna, in order to receive the signals from it. The apparatus furthermore comprises a frequency converter, preferably a mixer, which is associated with the selection device. Furthermore, a device which provides a reference frequency is associated with the frequency converter, which device comprises, for example, an oscillator which is preferably stabilized by a PLL circuit. The oscillator can, in particular, be tuned, such that the frequency of the received signals can be converted to a defined intermediate frequency. The apparatus also comprises an assembly, preferably a filter assembly, which contains a first frequency filter which defines a first and a second frequency range. In consequence, the bandwidth required for tuning the oscillator when switching over from the reception mode to the tracing mode is advantageously reduced. The first frequency filter in this case preferably comprises a first and a second filter, for example bandpass filters. Furthermore, the apparatus comprises a receiving device for the signals, which receiving device comprises, for example, a demodulator for demodulating the signals. The pass band of the first filter is preferably matched to the intermediate frequency, and the pass band of the second filter is matched to the difference between the intermediate frequency and the separation between associated transmission and reception channels. In this advantageous refinement of the invention, the oscillator is tuned such that frequencies of a defined reception channel are converted to the intermediate frequency, and thus essentially fall in the pass band of the first filter, in which case the second filter is matched to the associated transmission channel for the same oscillator frequency. In consequence, there is advantageously no need to retune the oscillator when changing between the reception and tracing mode, in order to test the associated transmission channel for freedom or occupancy.
In one particular embodiment, the intermediate frequency is equal to the separation between associated transmission and reception channels. In this embodiment, a direct conversion takes place in the tracing mode, since the reference frequency is approximately equal to the mid-frequency of the corresponding transmission channel.
The selection device preferably comprises a second frequency filter which is defined, for example, by a third and a fourth filter and comprises, for example, a first switch. The third and the fourth filter are preferably in the form of bandpass filters and are matched to the reception and transmission band, respectively, preferably in accordance with the GSM Standard. When the apparatus is in the reception mode, the signals are passed via the third and first filters from the antenna to the receiving device. For this purpose, the oscillator is tuned to a frequency which corresponds to the sum of the frequency of the reception channel and the intermediate frequency, in order to convert the received frequency to the intermediate frequency. When the apparatus is in the tracing mode, the signals are essentially fed via the fourth and second filters from the antenna to a meter, by means of which the field strength of the received signals can be measured.
An oscillator frequency range which is less than that for the prior art, and with the width of the reception band, is preferably advantageously sufficient.
Furthermore, the bandwidths of the first and second filters can be matched independently of one another, according to the invention, this being a further advantage. It is particularly advantageous to choose the width of the frequency band of the second filter such that a plurality of channels are covered.
The changeover process advantageously requires only a short time since, preferably, no retuning of the oscillator takes place. In this case, it is particularly advantageous to choose the width of the frequency band of the second filter such that three channels are covered at the same time. It is thus possible to determine in one step whether the corresponding transmission channel and the lower and upper adjacent channels are free. This is particularly advantageous since the adjacent channels are also preferably kept free, in order to transmit without interference.
The receiving device is preferably associated with the first filter, and the apparatus comprises a meter by means of which the field strength of the received signals can essentially be measured, and which apparatus is associated with the second filter. The bandwidth for the first filter is preferably essentially one channel width, so that signals in the defined channel are received from the reception band. By switching the selection device over from the third to the fourth filter, the apparatus is tuned to the transmission channel associated with the previously received channel, without having to retune the oscillator. In this state, the meter can measure the signal level of the associated transmission channel and, preferably, of the adjacent channels. After a very short time, in particular less than 500 xcexcs, the first switch can be switched back to the third filter again, in order to change to the reception mode once again.
In an alternative development of the invention, the assembly comprises a second switch, which defines a first and a second state. One connection of the switch is in each case connected to the receiving device, to the first filter and to the second filter, so that the receiving device is connected to the first filter when the switch is in the first state, and the receiving device is connected to the second filter when the switch is in the second state. In the receiving mode, the first and second switches are each in the first state, so that signals from the reception band are received by the receiving device via the third and first filters. In order to switch over to the tracing mode, the first and second switches are both changed to the second state, preferably at the same time, so that the receiver receives signals in the transmission band via the fourth and second filters. The receiver preferably has an output for connection of a field strength meter or field strength indicator, so that, in the tracing mode, it is possible to use this field strength meter to confirm whether the transmission channel associated with the reception channel and, preferably, its adjacent channels, are occupied. This switching process of the two switches from the respective first to the respective second states and back also advantageously requires only a very short time, since the oscillator need not necessarily be retuned in this case either.
With respect to the GSM Standard, the time is divided into time intervals, so-called time slots of about 500 xcexcs, so that it is advantageously possible while a radio connection is in existence to change from the reception mode to the tracing mode and back, if a time interval is not occupied, since the changeover process advantageously lasts for a shorter time than the time interval. If a part of, or the entire, transmission band is scanned in the tracing mode, the oscillator is essentially tuned through only half the width of the transmission band so that the scanning of a plurality of channels also takes less time.